One conventional example of means for obtaining image information from both sides of a document sheet can be found in Japanese Laid-Open Patent Application Tokukaihei 11-136444/1999 (published on May 21, 1999), which discloses a method for simultaneously obtaining image information from both sides of a document sheet by oppositely placing two line sensors on both sides of the document sheet.
However, the image scanning device for simultaneously obtaining information from both sides of the document, such as the one disclosed in the publication above, is disadvantageous in such a regard that, when irradiation light is simultaneously emitted with respect to the same position of the front and rear surfaces of the document, a line sensor placed on one side of the document scans the image information (such as characters) of both the front and rear surfaces of the document. As a result, the device fails to respectively scan the image information from the front and rear surfaces of the document.
One example aimed at solving this problem can be the scanner device disclosed in Japanese Laid-Open Patent Application Tokukaihei 6-253097/1994 (published on Sep. 9, 1994).
The scanner device disclosed in this publication has such an arrangement that two CCD image sensors are oppositely placed fixedly on both sides of a document sheet, while simultaneously emitting two light beams having different wavelengths respectively to the front and rear surfaces of the carried document. This arrangement prevents each of the two CCD sensors from scanning image information from both sides of the document, and allows each CCD sensor to desirably obtain the target image information from each side of the document.
However, since the image scanning device of the first publication scans the document by line sensors, the device requires a mechanism for moving two line sensors, so as to allow them to scan both sides of the document at the same time. This offers a hindrance to realize miniaturization and reductions in thickness and weight of the device. Further, since the device performs simultaneous scanning, it is necessary to block the transmitted light. Furthermore, scanning by line sensors is time consuming.
Meanwhile, as to the second publication, the scanner device is made of a CCD image sensor, which needs focusing, and therefore the device requires an optical system such as a lens, a light path or the like. This offers a hindrance to realize miniaturization and reductions in thickness and weight of the device, and also arises a problem of displacement of the optical system caused by vibration.
Further, both of the foregoing two image scanning devices are only capable of scanning of a document in the form of a sheet, and therefore incapable of two-sided scanning for a thick book etc.